Known methods of coding packet headers rely on time correlation techniques. The utilisation of a packet switching network is linked to the bit-rate. The utilisation also depends upon the ratio of data time to wasted time, that is to say the ratio of the time the network is transmitting data to the time data is not being transmitted. In the time domain, the wasted time is made up of the time taken up with header transmission (the header of a packet occupying a separate time slot at the head of the data of that packet), and by the guard band transmission time, the guard band being the separation between adjacent packets which is essential to avoid overlap of the packets due to dispersion during transmission.